Conventionally, a method used for a display apparatus such as a head-mounted display (HMD) is to draw an image directly onto the retina by scanning laser beams two-dimensionally (hereinafter, described as a laser-scanning method) (For example, see Patent Reference 1). The display apparatus according to the laser scanning method is also known as: retinal scanning display (RSD), retinal irradiation display, retinal direct-draw display, laser scanning display, direct-view-type display, virtual retinal display (VRD), and so on.
FIGS. 1A and 1B show an example of a structure of a goggle-type HMD. The HMD shown in FIGS. 1A and 1B has goggle frames equipped with: light sources 101 and 110 which emit laser beams; wavefront shape changing units 102 and 109 which control the wavefront of the laser beam; and scan units 103 and 108 which scan laser beams in a two-dimensional direction. The laser beams are projected by the scan units 103 and 108 onto the goggle lenses 11 and 12, and then reflected by deflection units 104 and 107 provided on the surface of the goggle lenses. Then, the laser beams enter a user's eyes to form an image on the retina. Here, a half mirror or a hologram optical element (HOE) is used for the deflection units 104 and 107 so that the user can visually recognize both the external world and the image drawn with the laser beams at the same time. In addition, for the scan units 103 and 108, a mirror device is used which oscillates a single-plate mirror in a uniaxial or biaxial direction to thereby scan the laser in a two-dimensional direction.
In addition, for an HMD that displays an image onto both eyes, there is a prior-art example of increasing virtual resolution by changing the number of pixels of the image to be displayed onto both eyes. In this prior-art example, by displaying pixels which complement each other, for example, displaying odd lines onto the right eye and even lines onto the left eye, it becomes possible to present, to the user, an image for which display pixels on the right and left are synthesized.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. H10-301055    Patent Reference 2: Japanese Unexamined Patent Application Publication No. H9-061776